I have no idea how many generations there were between Halbert and Aubrey, although I can tell you that Malcolm and Aubrey's mother was born about a century prior to the events in BoP, and Halbert was exiled to Camberia maybe 100 to 150 years before that, so maybe you can speculate from there.

Similarly, while I know that Morgan Haldane is a cousin to Maureen and her daughter Eilonwy, I don't know the exact relationship between his mother and the present Duke, Maureen's father. Celeste would probably have been too young to be the Duke's sister unless she was a very much younger sister, since she would have been from around Maureen's generation. I'm thinking she might have been descended from one of the Duke's siblings, therefore Maureen's first cousin. Or possibly she was a descendant of one of the Duke's father's siblings. For that matter, Fr Devlin's bishop (the auxiliary bishop of Rhemuth) is Sarah McLain, though I think her family line diverged from the ducal one quite early on, so any relationship to Maureen, etc., is very distant.